The present invention relates to improvements in apparatus primarily designed to measure the acoustic impedance or admittance of structures. In particular, it relates to apparatus designed to measure these values for the human external auditory canal.
Previously, the measurement of the acoustic impedance or the measurement of the reciprocal acoustic admittance has typically involved the measurement of such quantities at relatively low frequencies where each acoustic element was believed to be relatively small compared to the wavelength of sound and thus could be treated as a lumped constant, such as mass or stiffness. This is similar to describing electrical structures which are measured having a value of capacitance or inductance rather than being part of distributed circuits such as transmission lines.
For example, the medical screening for certain ear ailments involves in part, the measurement of the physical volume of the outer ear canal, the measurement of the effective volume of the ear canal and of the middle ear volume with the eardrum not tensioned, and a measurement of the volume change when the patient is exposed to acoustic or other stimuli.
The measurement of such volumes involves the measurement of the acoustic characteristics of the volume having dimensions which are very small compared to the wavelength of sound and air at the test frequency. If this Volume V was sealed but subjected to a Volume change .DELTA.V from some transducer, the resultant pressure change or sound pressure would be p=.gamma..times.P.sub.o .times..DELTA.V/V where P.sub.o is the ambient air pressure (near 10.sup.5 N/meter square at sea level) and .gamma. is the adiabatic coefficient (near 1.4 for air). If, for example, the relative volume change or .DELTA.V/V equal to 0.001% peak to peak, the resultant sound pressure level within that volume would be approximately 88 dB rms with respect to the standard reference level of 0.0002 dynes/cm.sup.2.
Practical instruments for the measurement of such small volumes of the ear canal amounting to values ranging between 0.2 and 5 cm.sup.3 have involved certain compromises which had to be reached because these volumes are comparable and perhaps even smaller than the volumes of the smallest miniature electroacoustic transducers available. Consequently, all these transducers had to be positioned outside the actual ear canal and be coupled to the ear canal by an ingenious arrangement of various concentric or separate tubes, ultimately terminating at a common seal which was positioned either on the surface of the entrance to the ear canal or within the entrance of the ear canal itself.
In practice also, at least two transducers had been involved, one of them, a microphone effectively measuring the sound pressure level and a second transducer acting as a generator of the volume displacement. In prior art, the output of the microphone would be measured after being amplified in a selective tuned circuit tuned to the measuring frequency, the output of that filter rectified and compared to a referenced voltage to provide a signal which would ultimately control the amplification of an electronic stage interspersed between a fixed frequency oscillator and the volume changing acoustic generator.
The signal at the input to the volume changing transducer often in the form of a miniature magnetic headphone-type of unit, would be measured by means of a voltmeter sensitive to alternating voltages. This voltmeter would be calibrated directly in terms of the acoustic volume connected to both the microphone and the other transducer.
These test probes containing both the microphone and the volume changing transducer have appeared in many forms, however all of these forms being ingenious arrangements so that the measurement of the acoustic volume was directly proportional to the aforesaid meter reading. Various tubes, be they concentric or placed side by side, connected to both microphone and the other transducer were used and these typically were of relativly small diameter and consequently, very easily contaminated by dirt and delicate to handle.
A second major disadvantage of these types of volume measuring probes has been their inaccuracy in maintaining linear relationship between the volume measured and the voltage applied to the transducer.
In order to avoid certain of the contamination and handling problems, probes have been designed which use a common duct leading to the end of the probe, however these probes have suffered not only from the presence of the added volume entering into the measurements, but also from the fact that resonance effects and other phenomena have caused a severe decrease in the possible accuracy of measurement.
Accordingly, the object of the apparatus that is about to be described is to overcome these disadvantages and to permit repeatable construction as well.
Other and further objects of the present invention will become apparent upon an understanding of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.